Log file reports SQL Error processing child comments

It looks like this is a bug related to caching in WordPress v4.6.1 which you can follow on Trac ticket #37696. A proper fix is scheduled for WordPress v4.6.2, but in the meantime, you could try the following workaround reported by Rarst, who created the issue on Trac:

Issue goes away if I disable Object Cache (Siteground’s Memcached).